The invention relates to a method of paving a sub-base with an asphalt concrete and the asphalt pavement produced thereby. More particularly, the invention relates to a method of asphalt paving a longitudinally extending continuous surface and creating a pavement having therein discrete dormant zones of potential fracture at predetermined positions.
Highways, roads, and sidewalks are frequently paved with a layer or mat of asphalt concrete which is laid over the surface of the sub-base. Typically, the asphalt concrete comprises asphalt cement combined with aggregates in a ratio of approximately 95 parts by weight of aggregate to approximately 5 parts by weight of liquid asphalt cement. The aggregate and liquid asphalt cement are heated and mixed to form an asphalt paving composition that includes some air voids.
The aggregate is usually a mixture of sand, gravel, and stone; the largest pieces of aggregate having a diameter equal to about 2/3 the thickness of the asphalt mat. Preferably, the aggregate has crushed particles to provide sharp edges in the gravel and stone which, when combined with the liquid asphaltic cement, create an aggregate interlock which improves the strength of the mat.
Other characteristics important in determining the quality of a particular aggregate mix are; first, the aggregate should be sound, i.e., it should be able to withstand a substantial amount of compression before it breaks apart; and second, the aggregate should have desirable wear characteristics, i.e., it should be resistant to friction and abrasion. Aggregates possessing these qualities can be very expensive.
Typically, the liquid asphalt cement is a low cost residue or by-product of the petroleum refinement process. In a newly refined state, it is generally very resilient, and when heated to the application temperature, becomes a thick, viscous liquid. The flexibility properties of the asphalt cement can be increased by blending less viscous oils to change the penetration index of the asphalt cement. This increases the cost of the asphalt cement and requires the use of fuels which may be better used as energy resources.
The usual process of laying a continuous longitudinally extending surface of asphalt mat such as on a highway or road, is well known in the art. The liquid asphaltic cement is heated to a temperature of approximately 250.degree.-300.degree. and mixed with the aggregate. The mixture is then spread evenly on a smooth sub-base and is compressed or compacted to provide a traveling surface that, when cooled to ambient temperatures, is initially very flexible and resistant to wear.
However, over time and with the change in seasons the asphalt mat is exposed to a wide range of changing temperatures. As the temperature decreases, the asphalt mat contracts causing random generally transverse cracking. When the temperature subsequently increases, the asphalt mat then expands. However, it expands randomly, and for reasons that are beyond the scope of this discussion, the mat does not return to its original size. Thus, when the mat expands there is less material to cover the same amount of sub-base and the cracks that developed during the period of decreased temperature remain. Though they appear randomly, empirical evidence suggests that the temperature induced cracks will occur approximately every 10 to 20 feet and can be as wide as 1/4 to 3/4 of an inch. Over a period of years, additional cracks will appear at greater frequency (i.e., closer longitudinal intervals) until some equilibrium is found between the stresses on the mat formed by the temperature changes and traffic, and the flexibility of the mat.
Another factor contributing to the random transverse cracking of the asphalt mat is the oxidation of the polymers in the liquid asphaltic cement. The oxidation causes asphalt cement to become more brittle and less flexible over time. As the asphalt cement oxidizes, it becomes more brittle and prone to cracking under the constant and random stresses induced by traffic and temperature changes.
Cracks formed due to these stresses typically have large widths (1/4 to 3/4 of an inch) and reduce the service life of the pavement. Large width cracks introduce an additional wear factor because they allow water to easily seep into and underneath the asphalt mat in large volume. Because water expands when it freezes, it adds to the stress on the mat during periods of freezing and thawing. Therefore, wide cracks, which heretofore have been unavoidable, create a pavement that requires a high amount of expensive maintenance and one that deteriorates at a higher rate than one where cracks are narrow or non-existent.
Extensive efforts have been made over more than a century to solve this problem. It is known that by chemically modifying the asphalt cement it is possible to make the resulting pavement more flexible. However, modifying the asphalt cement substantially increases the cost of the asphalt cement. While these modifications provide some benefit, the durations of their usefulness is limited as they are still prone to oxidation and the resulting brittleness.
In order to improve safety government specifications generally require an asphalt concrete that has a bulk specific gravity and density when compacted that will resist rutting and potholing, thus precluding some desirable chemical modifications of the asphalt cement. Such an asphalt pavement has excellent wear and anti-rutting characteristics but unfortunately it acquires a degree of inflexibility that makes it highly susceptible to developing wide cracks.
Prior efforts to solve the problem of random transverse cracking in asphalt have also resulted in the use of what is termed the saw and seal technique typically used on existing concrete roadways. This technique makes use of the knowledge that reflective cracking will occur when an asphalt mat is placed over an existing concrete pavement. Accordingly, once the asphalt mat has been paved and has cooled, the mat is then sawed through in a transverse direction creating joints in the asphalt pavement directly over the cracks in the underlying concrete pavement to dictate where cracking will occur. The joints are then sealed with some type of silicone or tar sealant to prevent water from seeping into the cracks where it would develop into an additional wear factor as the water expands and contracts due to seasonal freezing and thawing. This is an effective way to contour cracking. However, saw and seal is a very time consuming and expensive procedure, so expensive in fact that it is not used on most highways.
Another effort to provide a crack resistant longitudinally extending asphalt mat has been to employ polymer science to improve the flexibility of the asphalt mat through chemically engineered additives. Chemical additives, in the volume needed are extremely costly and the resulting pavement is nevertheless still subject to the oxidation and plasticization which, over time, takes place in the conventional asphalt mat to make it less flexible and more subject to cracking.
Thus, a need exists to provide an asphalt mat that is durable in that it is able to hold together and resist potholing, stable in that it is capable of resisting rutting, and yet, in an efficient, low-cost manner, minimizes the development of the wide random cracks that heretofore occurred in highways, roads, and sidewalks in an efficient low cost manner. Further, an asphalt mat is needed that minimizes moisture penetration even as the mat develops stress fractures and that is less dependent on high quality expensive aggregate to provide strength to resist cracking in the mat. It is also desirable to be able to attain the aforementioned characteristics in an asphalt pavement to using an inexpensive asphalt cement.